Chemicals that colour fruit and vegetables fight cancer and could provide potent new drugs to combat the disease, scientists revealed yesterday.

Experiments on rats and human cells have shown that anthocyanins, found in red, purple and blue-coloured plants, can significantly slow colon cancer.

Altering the structure of some of the chemicals made them even more potent, scientists found.

Plants used in the tests included grapes, radishes, purple corn, chokeberries, bilberries, purple carrots and elderberries.

Study leader Dr Monica Giusti, from Ohio State University in the US, said: "These foods contain many compounds, and we're just starting to figure out what they are and which ones provide the best benefits."

Purple corn extract was found to be the most potent, taking just 14 micrograms per millilitre of solution, to reduce tumour cell numbers by 50 per cent.